Deep Simplicity: Chaos, Complexity and the Emergence of Life by John Gribbin

Author:John Gribbin [Gribbin, John]
Language: eng
Format: epub
Tags: Science, Chaotic Behavior in Systems, General, Language Arts & Disciplines, Library & Information Science, Popular Science
ISBN: 9780141042213
Google: MlpJOXbkZ5oC
Publisher: Penguin Books Limited
Published: 2009-08-26T19:00:00+00:00

Figure 4.2 The patterns produced on the body of an animal by self-organized chemical processes during the development of an embryo (see below) are related to the size of the animal. The chemical diffusion processes produce stripes on smaller areas, and spots in larger areas.

One of the key features of the model, though, is that the kind of pattern that forms over the surface of an animal does not depend on the size and shape of the adult, but on the size and shape of the embryo at the time the Turing process is at work. Clearly there is some correlation with the size of the adult, because from very soon after conception elephant embryos tend to be larger at the same stage of development than mouse embryos; but the significance of the embryo size is beautifully highlighted by the differences in the stripes of the two kinds of zebra, Equus burchelli and Equus grevyi. The former has fewer and broader stripes than the latter, making them distinctly different when seen alongside each other, even though the adults are roughly the same size. By counting the number of stripes in each case, and taking account of the way the pattern had been distorted by the growth of the animal, in the 1970s J. B. L. Bard showed that the pattern seen on burchelli must be laid down on the embryo when it is twenty-one days old, while the pattern seen on grevyi must be laid down on the embryo when it is five weeks old. This was known before Murray came up with his mathematical model of the Turing effect, but the differences exactly correspond to the predictions of the model, with broader stripes corresponding to an earlier diffusion of the actuator and inhibitor across the surface of the embryo. The dramatic way in which the genetics and the environment (‘nature’ and ‘nurture’) combine in this way was highlighted early in the year 2002, when news was released of the birth of the first cloned kitten. Because the pattern of colours on multicoloured animals results from a combination of their genetic inheritance and events in the womb (including, for example, the amount of nutrition received by the developing embryo), the kitten did not have exactly the same pattern of markings in her fur as her mother, even though the two animals had identical DNA.

This also brings us on to the significance of all this to our understanding of evolution. The visible differences between the patterns in the two species of zebra are produced simply by changing the time at which the Turing effect is at work in the embryo. As far as we know there is no evolutionary advantage in either pattern in this particular case (not every feature of anatomy has to be adaptive). But if there were an advantage in having narrower (or broader) stripes, perhaps in terms of providing better camouflage, it is easy to see how natural variations from one individual to another could provide the

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